Guile hasn't had an official repository. That's why guile hasn't accumulated thousands of modules. Since the official gnu guix repository is mainly for GuixSD, it makes sense to make another guix repository for guile modules to be used on multiple linux distributions. I'm not sure if guix works on windows and Mac OS, too.
If you made it possible to add third party guix repository URLs, people would add third party repositories later without upfront coordination with guix developers.
